1. Field of the Invention
The present invention pertains to cement compositions formed from regular and microfine ground slag cements and oil based drilling fluids comprising water in oil emulsions, for controlling lost circulation and other cement plugging applications in wells.
2. Background
In drilling and completing oil and gas wells, for example, it is well-known practice to pump Portland cement mixtures into the wellbore to form a seal between the well casing and the earth formation, to control lost circulation of the drilling fluid and to plug portions of the wellbore for various purposes including forming a foundation for side-tracking or drilling a deviated wellbore from the original wellbore. Conventional water-based Portland cement slurries are often pumped into the well to control lost circulation or to otherwise set a "cement plug" in the wellbore. Typically, an excess volume of the cement mixture is pumped into the well to compensate for the adverse effects of contamination of the cement mixture by the presence of drilling fluids or "muds" particularly oil-based muds, in the wellbore The excess volume of cement is used to provide enough settable cement for a competent plug in contemplation that a large portion of the cement will become contaminated and not be a settable composition. Moreover, the oil wetting type surfactants that are used to formulate oil-based drilling muds also impede cement hydration reactions. The oil wet surfaces of the cement particles cannot sufficiently mix with hydration water to provide adequate strength development. The cement dilution that results from drilling mud contamination also reduces the compressive strength of that portion of the cement which will actually harden or set.
Quite often, a cement plug is desired to be placed in the wellbore a considerable distance above the bottom of the well. These plugs are particularly prone to failure from drilling mud contamination since the Portland cement slurry is generally more dense than the drilling mud and undergoes gravitational movement which forces intermixing of the drilling mud with the cement slurry, again resulting in a cement composition which has inadequate compressive strength or may simply, through gravitational forces, fall far below the target position of the plug in the wellbore. Multiple, expensive attempts to set cement plugs often result from prior art problems with conventional Portland cement based slurries when exposed to oil-based drilling fluids.
Still further, so-called "pills" or slugs of viscous fluid have been used to minimize the falling or unwanted separation of relatively dense cement slurries to prevent the slurries from falling through less dense water-based drilling fluids or completion fluids. The viscous fluid pills are designed to serve as a platform for setting conventional Portland cement plugs above the well bottom. The goal in this case is to reduce potential fluid movement and contamination of the cement mixture. While such viscous fluid pills are often used, they are not consistently successful. Viscous pills work very poorly in deviated wellbores since the different fluids in the well tend to slip past each other. Moreover, the use of viscous fluid pills in wells which are occupied by oil-based mud is even less likely to be successful since the oil-wetting properties of these muds reduce the desirable properties of the pills. Desirable rheological characteristics of viscous pills for wells are extreme apparent viscosity, very high yield point, high gel strength and a high rate of gel strength development. In some applications it is even more advantageous if the viscous pill forms a true solid with measurable compressive structural strength. These viscous pill properties are generally obtainable when pumping viscous pills into wells occupied by water-based muds by adding concentrations of bentonite, certain polymers and the use of Portland cement. However, the development of viscous pills with a desirable rate of gel strength development (progressive gelation) and compressive strength have not been obtainable in an oil-based mud environment. In this same regard, squeeze cementing and the placement of cement plugs in oil-based muds using conventional Portland cements are often unsuccessful.
U.S. Pat. No. 2,890,752, issued Jun. 16, 1959 to Crone, et al, describes a squeeze cementing composition wherein conventional Portland oil well cement is mixed with kerosene, diesel oil or crude oil, together with octyl phenol and ethylene oxide as a surfactant to provide a low-pressure squeeze cement composition to block water-producing zones in an earth formation. U.S. Pat. No. 3,861,468 to Curtice, et al, issued Jan. 21, 1975, describes a method of treating permeable, unconsolidated, sand-containing formations wherein Portland cement, a hydrocarbon oil such as kerosene or diesel oil and refined motor oil and an aqueous carrier medium is pumped into a wellbore and into a formation to be consolidated. U.S. Pat. No. 3,862,663 to Curtice, et al, and issued Jan. 28, 1975, also describes a method for treating an oil-containing incompetent earth formation with a composition which includes a refined lubricating oil fraction, an oil-wetting agent and Portland cement in a water carrier liquid. U.S. Pat. No. 4,124,075, issued November 7, 1978 to Messenger, describes a water-based Portland cement slurry which is placed against an evaporite formation section after an oil-wetting liquid is flowed through and in contact with the section. In the publication entitled "Lost Circulation" Pennwell Publishing Company, Tulsa, Oklahoma, 1981Messenger describes a composition for forming cement plugs which includes bentonite, API Class G Portland cement and diesel oil. A publication entitled "Case Histories Regarding the Application of Microfine Cements" by Heathman and East, Jr., IADC/SPE 23926Society of Petroleum Engineers, Richardson, Texas, describes the use of so-called microfine cements to prepare casing leaks, and stop water encroachment into a well using cement slurries with small particle size (8 to 15 microns) cement materials in conventional water-based slurries. An article entitled "Well Cementing Achieved with Slug of Converted Drilling Fluid", Offshore/Oil Man, November 1992, describes the use of hydraulic blast furnace slag added to water-based drilling fluid to serve as a well cement mixture. An anonymous disclosure in the publication Research Disclosure, June 1992, describes a cement composition containing blast furnace slag in microfine particle size, diesel or light oil, surfactant, slag activator and water which is batch mixed and pumped into a wellbore as a fluid loss or fluid production control agent. This mixture is designed to set only upon contact with produced water in the wellbore and any portion of the mixture that does not contact the produced water in the wellbore remains fluid and is circulated out of the well.
With this background of problems and the state-of-the-art in developing cement plugs and viscous fluid "pills" for use in wells occupied by oil-based drilling muds, a surprising discovery has been made in accordance with the present invention.